There have been made a number of proposals to use a silicon thin film as a constituent element in scanning circuits of image-reading devices such as lengthy uni-dimensional photosensor, large sized two-dimensional photosensor, etc., or in operation circuits of image-display devices using liquid crystal (LC), electroluminescent cell (EL) or electrochromic material (EC). Various studies have been made on hydrogenated amorphous silicon thin films or polycrystal silicon thin films to be used as such silicon thin film.
However, as for such known amorphous silicon thin film, there are problems that its effective electric carrier mobility (.mu. eff) is a small value of about 0.1 cm.sup.2 /V.sec. On the other hand, the .mu. eff, which is demanded for the constituent layer element in the scanning circuit or the operation circuit of recently developed high-speed and high functional image-reading device or image-display device, is 50 to 100 cm.sup.2 /V.sec.
There are problems also for such known polycrystal silicon thin film that although its .mu. eff is rather large compared to that of said amorphous silicon thin film, it is still insufficient, and in addition to this, it is difficult to obtain a large size polycrystal silicon film of uniform quality. As one of the reasons for this, there is present a localized state density of about 10.sup.18 cm.sup.-3 at the grain boundary. When there is present such defect for the polycrystal silicon thin film, there will occur problems as mentioned below.
That is, (a) such state density at the grain boundary hinders the formation of a channel to cause heightening in the threshold voltage, and (b) such state density also hinders electric carriers to be transported and causes decrease in the field effect mobility.
In order to eliminate the problems relating to the foregoing polycrystal silicon thin film, there have been proposed a method of subjecting the resultant polycrystal silicon thin film to hydrogen plasma treatment to decrease such state density, and another method of subjecting the resultant polycrystal silicon thin film to laser annealing or electron beam annealing to widen the sizes of grains therein thereby decreasing the existing grain boundaries, as disclosed in IEEE Electron Device Letters, vol. EDL-1, No. 8, p. 159 (Aug. 1980). However, any of such proposed methods is not sufficient for solving the foregoing problems. That is, in the case of the above hydrogen plasma treatment, it is difficult to sufficiently decrease such state density at the grain boundary. And in the case of the laser annealing and in the case of the electron beam annealing, there are problems that it is difficult to eliminate an undesired influence caused by the surface state. It is also difficult to form a large size polycrystal silicon thin film having uniform characteristics in the case of preparing a large size thin film transistor.
Other than the above proposals, there has been proposed a method of forming a polycrystal silicon thin film while maintaining a substrate at relatively low temperature by way of associating or reacting different radicals in vapor phase as disclosed in Japanese Patent Unexamined Publication No. Sho. 62-40717. According to this proposed method, it is possible to incorporate hydrogen atom into a film while it being formed, and because of this, the defect of the grain boundary may be compensated with said hydrogen atom.
However, even for this method, there are problems that the hydrogen atoms as incorporated cause an increase in the stress in a film being formed and because of this, there often occurs peeling-off of a film during the film forming process.
In view of the above, it was difficult to immobilize a semiconductor device using a non-single-crystal silicon thin film as the constituent element layer in the past.